System and method of depth imaging based on rolling shutter cmos image sensor and down conversion techniques

ABSTRACT

A vehicle, imaging system and method of determining a range of an object to a vehicle. The imaging system includes a light source, a receiver and a processor. The light source is configured to transmit a source signal at a source frequency at an object, wherein the source signal is reflected from the object to create a reflected signal. The receiver includes a sensor array and is configured to modulate the reflected signal at a mixing frequency to generate a down-converted signal and record the down-converted signal at the sensor array. The processor is configured to determine a range of the object to the vehicle using the down-converted signal.

INTRODUCTION

The subject disclosure relates to imaging systems and, in particular, toa system and method of depth imaging using signal down-conversion.

Depth imaging is useful in various fields, such as for determining alocation of an object within an environment being traversed by avehicle. One method of depth imaging includes indirect time-of-flightimaging. In indirect time-of-flight imaging, a scene is illuminated withan amplitude modulated continuous wave (AMCW) source signal and a phasedifference between the source signal and a reflected signal is measured.Typically, the AMCW source signal has a frequency in the Megahertzrange. Standard image sensors are bandwidth limited when capturing thephase of such high frequency signals. Accordingly, it is desirable touse a signal processing technique to capture information at highfrequencies using standard image sensors.

SUMMARY

In one exemplary embodiment, a method of determining a range of anobject at a vehicle is disclosed. A source signal is transmitted fromthe vehicle at a source frequency at the object, wherein the sourcesignal is reflected from the object to create a reflected signal. Thereflected signal is modulated using a mixing frequency to form adown-converted signal. The down-converted signal is recorded at a sensorarray of the vehicle. The range of the object to the vehicle isdetermined using the down-converted signal.

In addition to one or more of the features described herein, modulatingthe reflected signal includes modulating an intensity of the reflectedsignal at the mixing frequency using a modulator between the object andthe sensor array, in one embodiment. In another embodiment, modulatingthe reflected signal includes modulating a quantum efficiency of thesensor array at the mixing frequency. The down-converted signal isrecorded at a pixel cell comprising four rows, each row having fourpixels. The four pixels of each of the four rows are binned to obtainfour binned pixels, each binned pixel having a row signal amplitude anda time stamp. The method further includes determining at least one of aphase of the reflected signal from the row signal amplitudes and anamplitude or magnitude of the reflected signal from the row signalamplitudes. The method further includes determining a time-of-flight ofthe reflected signal and a range to the object from the time-of-flight.

In another exemplary embodiment, an imaging system of a vehicle isdisclosed. The imaging system includes a light source, a receiver and aprocessor. The light source is configured to transmit a source signal ata source frequency at an object, wherein the source signal is reflectedfrom the object to create a reflected signal. The receiver includes asensor array and is configured to modulate the reflected signal at amixing frequency to generate a down-converted signal and record thedown-converted signal at the sensor array. The processor is configuredto determine a range of the object to the vehicle using thedown-converted signal.

In addition to one or more of the features described herein, the imagingsystem further includes a modulator configured to modulate an intensityof the reflected signal at the mixing frequency to generate thedown-converted signal. In another embodiment, a quantum efficiency ofthe sensor array is adjusted at the mixing frequency to generate thedown-converted signal. The processor is further configured to generate apixel cell comprising four rows, each row having four pixels, and recordthe down-converted signal using the pixel cell. The processor is furtherconfigured to bin the four pixels of each of the four rows to obtainfour binned pixels, each binned pixel having a row signal amplitude anda time stamp. The processor is further configured to determine at leastone of a phase of the reflected signal from the row signal amplitudesand an amplitude of the reflected signal from the row signal amplitudes.The processor is further configured to determine a time-of-flight of thedown-converted signal and a range to the object from the time-of-flight.

In yet another exemplary embodiment, a vehicle is disclosed. The vehicleincludes a light source, a receiver and a processor. The light source isconfigured to transmit a source signal at a source frequency at anobject, wherein the source signal is reflected from the object to createa reflected signal. The receiver includes a sensor array and isconfigured to modulate the reflected signal at a mixing frequency togenerate a down-converted signal and record the down-converted signal atthe sensor array. The processor is configured to determine a range ofthe object using the down-converted signal.

In addition to one or more of the features described herein, a modulatoris configured to modulate an intensity of the reflected signal at themixing frequency to generate the down-converted signal. In anotherembodiment, a quantum efficiency of the sensor array is adjusted at themixing frequency to generate the down-converted signal. The processor isfurther configured to generate a pixel cell comprising four rows, eachrow having four pixels, and record the down-converted signal using thepixel cell. The processor is further configured to bin the four pixelsof each of the four rows to obtain four binned pixels, each binned pixelhaving a row signal amplitude and a time stamp. The processor is furtherconfigured to determine at least one of a phase of the reflected signalfrom the row signal amplitudes and an amplitude of the reflected signalfrom the row signal amplitudes.

The above features and advantages, and other features and advantages ofthe disclosure are readily apparent from the following detaileddescription when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, advantages and details appear, by way of example only,in the following detailed description, the detailed descriptionreferring to the drawings in which:

FIG. 1 shows an autonomous vehicle in an exemplary embodiment;

FIG. 2 shows a schematic diagram of a depth imaging system of thevehicle, in an embodiment;

FIG. 3 shows a block diagram illustrating operation of the depth imagingsystem, in an embodiment;

FIG. 4 illustrates an operation of a signal processor of the depthimaging system;

FIG. 5 illustrates a use of a pixel cell to process pixels fordetermining depth and intensity information from pixel measurements;

FIG. 6 illustrates operation of an image processor of the depth imagingsystem;

FIG. 7 shows a schematic diagram of an imaging system in an alternateembodiment;

FIG. 8 shows a pixel of a modulated sensor array of the alternateimaging system, in an embodiment;

FIG. 9 shows a graph of quantum efficiency for PN junctions havingdifferent doping concentrations; and

FIG. 10 shows a block diagram illustrating operation of the alternateimaging system.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, its application or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

In accordance with an exemplary embodiment, FIG. 1 shows an autonomousvehicle 10. In an exemplary embodiment, the autonomous vehicle 10 is aso-called Level Four or Level Five automation system. A Level Foursystem indicates “high automation”, referring to the drivingmode-specific performance by an automated driving system of all aspectsof the dynamic driving task, even if a human driver does not respondappropriately to a request to intervene. A Level Five system indicates“full automation”, referring to the full-time performance by anautomated driving system of all aspects of the dynamic driving taskunder all roadway and environmental conditions that can be managed by ahuman driver. It is to be understood that the system and methodsdisclosed herein can also be used with an autonomous vehicle operatingat any of the Levels One through Five.

The autonomous vehicle 10 generally includes at least a navigationsystem 20, a propulsion system 22, a transmission system 24, a steeringsystem 26, a brake system 28, a sensor system 30, an actuator system 32,and a controller 34. The navigation system 20 determines a road-levelroute plan for automated driving of the autonomous vehicle 10. Thepropulsion system 22 provides power for creating a motive force for theautonomous vehicle 10 and can, in various embodiments, include aninternal combustion engine, an electric machine such as a tractionmotor, and/or a fuel cell propulsion system. The transmission system 24is configured to transmit power from the propulsion system 22 to two ormore wheels 16 of the autonomous vehicle 10 according to selectablespeed ratios. The steering system 26 influences a position of the two ormore wheels 16. While depicted as including a steering wheel 27 forillustrative purposes, in some embodiments contemplated within the scopeof the present disclosure, the steering system 26 may not include asteering wheel 27. The brake system 28 is configured to provide brakingtorque to the two or more wheels 16.

The sensor system 30 senses an object 50 in an exterior environment ofthe autonomous vehicle 10 and determines parameters such as a positionand/or velocity of the object 50 with respect to the autonomous vehicle10. Such parameters can be provided to the controller 34 fornavigational purposes. Operation of the sensor system 30 is discussedwith respect to FIG. 2, in an embodiment. The sensor system 30 includesadditional sensors, such as digital cameras, for identifying roadfeatures, etc.

The controller 34 builds a trajectory for the autonomous vehicle 10based on the output of sensor system 30. The controller 34 can providethe trajectory to the actuator system 32 to control the propulsionsystem 22, transmission system 24, steering system 26, and/or brakesystem 28 in order to navigate the autonomous vehicle 10 with respect tothe object 50.

The controller 34 includes a processor 36 and a computer readablestorage device or storage medium 38. The computer readable storagemedium includes programs or instructions 39 that, when executed by theprocessor 36, operate the autonomous vehicle based on sensor systemoutputs. The storage medium 38 may further include programs orinstructions 39 that when executed by the processor 36, determines astate of object 50 in order to allow the autonomous vehicle to navigatewith respect the object.

FIG. 2 shows a schematic diagram of a depth imaging system 200 in anembodiment. The depth imaging system 200 includes a light source 202 anda receiver 208. The light source 202 can be an amplitude modulatedcontinuous wave (AMCW) light source that generates an AMCW source signal204 and transmits the AMCW source signal 204 at an object 50. In variousembodiments, the frequency of the AMCW source signal 204 is in theMegahertz frequency range. The AMCW source signal 204 is reflected froman object 50 within the scene to generate a reflected signal 206. Thereflected signal 206 is received at the receiver 208.

In an embodiment, the receiver 208 is a digital camera including asensor array 210 located in an imaging plane of the digital camera and ashutter or modulator 212 in front of the sensor array 210. The sensorarray 210 includes an array of pixels that generate a voltage in directproportion to an amount of light incident on the pixel. The modulator212 is modulated between a high-transmissivity state and alow-transmissivity state at a selected mixing frequency. The reflectedsignal 206 passes through the modulator 212. The modulator 212 modulatesthe reflected signal 206 to create a mixed signal. The mixed signal isreceived at the sensor array 210. In various embodiments, the modulator212 down-converts a frequency of the reflected signal 206 to generatethe mixed signal. The mixed signal is therefore a down-converted signal.

A system controller 214 synchronizes operation of the light source 202,receiver 208 and modulator 212, allowing the mixed signal received atthe receiver 208 to be compared to the source signal 204. A signalprocessor 216 reads voltage values at the pixels of the sensor array 210and determines from these voltage values phase and amplitude values ofthe mixed signal. An image processor 218 then determines a depth and/orimage of the object 50, giving a three-dimensional image of the object.

FIG. 3 shows a block diagram 300 illustrating operation of the depthimaging system 200, in an embodiment. The block diagram 300 includes thelight source 202, receiver 208, modulator 212, sensor array 210, signalprocessor 216 and image processor 218.

The light source 202 generates the source signal 204 at a sourcefrequency f_(s). The source signal 204 has a waveform F_(TX) asindicated in Eq. (1):

F _(TX) =P·[cos(2πf _(s) t−ϕ ₀)+1]  Eq, (1)

where P is the original power of the source signal 204, f_(s) is thefrequency of the source signal, and ϕ₀ is the original phase of thesource signal. A reflected signal 206 created by reflection of thesource signal 204 from the environment or object 50 is incident at thereceiver 208. The sensor array 210 of the receiver 208 forms atwo-dimensional array of pixels having a dimension of N×M, where Nis thenumber of rows and M is the number of columns. The reflected lightreceived at a pixel (n, m) of the sensor array 210 is received with atime delay t_(d) ^(n,m). Thus, for a selected pixel (n, m), thereflected signal 206 received at the pixel has a waveform F_(RX) ^(n,m)in given by Eq. (2):

F _(RX) ^(n,m) =A _(n,m)·[cos(2πf _(st)(t−t _(d) ^(n,m))−ϕ₀)+B_(n,m)]  Eq, (2)

where B_(n,m) is an amplitude of an oscillation of the reflected signal206 at the receiver 208 and B_(n,m) is related to an average power ofthe reflected signal 206. The modulator 212 modulates the reflectedsignal 206 using a mixing signal which has a waveform F_(MIX) indicatedin Eq. (3):

F _(MIX)=½·[cos(2πf _(MIX) t)+1]  Eq, (3)

where f_(MIX) is the frequency of modulation of the modulator 212. Themixing frequency f_(MIX) is selected to be close to the frequency of thesource signal f_(s), both of which are in the Megahertz range. Thedifference Δf between the mixing frequency and the frequency of thesource signal is given by Eq. (4):

f _(MIX) =f _(s) −Δf  Eq. (4)

and is generally in the range of kilohertz. The modulation of thereflected signal 206 using the modulator 212 generates a down-convertedsignal or a signal having a lower frequency. The main component of thedown-converted mixed signal F_(IN) received at the sensor array 210 isshown in Eq. (5):

$\begin{matrix}{F_{IN}^{n,m} = {\frac{A_{n,m}}{4} \cdot {\cos\left( {{2{\pi\Delta}\; f\; t} - \phi_{d}^{n,m} - \phi_{0}} \right)}}} & {{Eq},\;(5)}\end{matrix}$

Information about the object is held in the various parameters of thewaveform recorded at the sensor array 210. For example, the phase termϕ_(d) ^(n,m) holds information about a depth of the object and intensitymeasurements at the pixel (n, m) help determined reflectance of theobject.

FIG. 4 illustrates an operation of the signal processor 216. A timechart 400 shows relative times at which pixels of the sensor array 210are read by the signal processor 216, in an embodiment. Pixel rows areshown along the y-axis with time shown along the x-axis. N pixel rowsare shown with the first row (i.e., R1 at the top and the last row(i.e., RN) at the bottom.

For a selected row, (e.g., R1) a measurement time period for the row orpixels includes a reset period 402, an integration period 404 and areadout period 406. In the reset period 402, the pixel values are resetto an initialized value from their previous value. During theintegration period 404, the pixel is receptive to light and accumulatesa voltage value at the pixel indicative of light intensity at the pixel.During the readout period 406, the voltage at the pixel is read to aprocessor. Each row can have a timestamp associated with the measurementtime period, or more specifically, the readout period 406.

Observing the time chart 400, the pixel measurement time period for aselected row is offset in time by an amount that prevents overlap ofread out periods of adjacent rows. In one embodiment, the duration ofthe integration period T_(INT) for a row is set to the same duration asthe readout period T_(RO) for the row. Additionally, the integrationperiods and the readout periods for all rows are the same. Theintegration period 408 for a selected row (e.g., R3) is synchronizedwith the integration period 410 of the previous row (e.g., R2) such thatthe integration period 408 of the selected row begins when theintegration period 410 of the previous row ends. If the selected row isthe first pixel row (e.g., R1), then the integration period issynchronized to the integration period of the last pixel row (e.g., RN).As illustrated by the time chart 400, with this synchronization ofintegration periods, the readout period 412 of a selected row (e.g., R3)does not overlap either the readout period 414 of the previous row(e.g., R2) or the readout period 416 of the subsequent row (e.g., Row4).

The time chart 400 is separated into a plurality of time frames ( . . .Frame (k−1), Frame (k), Frame (k+1) . . . ). Each time frame lasts for aduration of time that allows each row (R1, . . . , RN) to be read once.Since rows are read sequentially, the duration of a time frame lastsfrom the time at which the readout of the first row (R1) is commenced tothe time at which readout of the last row (RN) is completed. Theduration of a time frame T_(INT) is therefore equal to the number ofpixel rows times the readout time for each row, as shown in Eq. (6):

T _(INT) =N _(ROWS) ·T _(RO)  Eq. (6)

where N_(ROWS) is the number of rows and T_(RO) is the readout time.

FIG. 5 illustrates a use of a pixel cell to process pixels fordetermining depth and intensity information from pixel measurements. Apixel cell 502 is defined having a selected dimension. In variousembodiments, a pixel cell 502 is a 4×4 cell (i.e, four rows (504 a, 504b, 504 c, 504 d), each row having 4 pixels). The pixel cell 502 acts asa window moving from pixel to pixel of the image sensor. The pixel cell502 accumulates information from the pixel rows in a sequence togenerate a lock-in pixel 508. Due to the size of the 4×4 pixel cell 502,three rows and three columns at the edges of the image sensor areneglected. Thus, an image sensor having an N×M array of pixels, producesa an (N−3)×(M−3) array of lock-in pixels.

Each of the rows (504 a, 504 b, 504 c, 504 d) of the pixel cell 502 hasan associated time stamp. The time stamp can be a time at which thereadout period begins, a time at which the readout period ends, or anyother time representative of the readout period, in various embodiments.

To process the pixel cell 502, each of the rows (504 a, 504 b, 504 c,504 d) of the pixel cell 502 is binned to obtain a row signal, therebyproducing four row signals S_(i) (i=1, . . . , 4), with each row signalhaving an associated time stamp. The row signal S_(i) can be a summationof pixel voltage values of an average of these values, in variousembodiments. The row signals 506 and time stamps are then associated toa lock-in pixel 508.

Graph 510 shows a relation between row signals S_(i) and timestampT_(i). Time is shown along the abscissa and signal intensity is shownalong the ordinate axis. Four time periods (T₁, T₂, T₃, T₄) are shown,each time period representing a readout period for the row. Row signalamplitudes (S₁, S₂, S₃, S₄) are shown within their respective readoutperiods. These row signal amplitudes can be used to determined depth andintensity information of the object, as discussed with respect to FIG.6.

FIG. 6 illustrates operation of the image processor 218. Graph 600 showsboth the source signal 204 and a reconstruction of the reflected signal206. Time is shown in 10⁻⁵ seconds along the abscissa and optimal poweris shown in Watts (W) along the ordinate axis. The image processor 218reconstructs the waveform of the reflected signal 206 from the rowsignals S_(i) shown in graph 510. The phase of the reflected signal 206can be determined from the row signal amplitudes S_(i) using Eq. (7):

$\begin{matrix}{\phi_{d} = {\tan^{- 1}\left( \frac{S_{1} - S_{3}}{S_{2} - S_{4}} \right)}} & {{Eq}.\mspace{14mu}(7)}\end{matrix}$

The time of flight t_(d) for between the transmission of the sourcesignal 204 and reception of the reflected signal 206 is related to thephase ϕ_(d), as shown in Eq. (8):

2πft _(d)=ϕ_(d)  Eq. (8)

The range of the object can then be determined using Eq. (9):

$\begin{matrix}{R = {\left( \frac{\phi_{d}}{2\pi} \right) \cdot R_{MAX}}} & {{Eq}.\mspace{14mu}(9)}\end{matrix}$

Where the maximum range is determined by Eq. (10):

$\begin{matrix}{R_{MAX} = \frac{c}{2f}} & {{Eq}.\mspace{14mu}(10)}\end{matrix}$

The amplitude of the reflected signal 206 can be determined from the rowsignal amplitudes S_(i), as shown in Eq. (11):

$\begin{matrix}{A = \left\lbrack \frac{\left( {S_{1} - S_{3}} \right)^{2} + \left( {S_{2} - S_{4}} \right)^{2}}{4} \right\rbrack^{1/2}} & {{Eq}.\mspace{14mu}(11)}\end{matrix}$

The average power of the reflected signal is given by Eq. (12):

$\begin{matrix}{B = {\frac{1}{4}{\sum_{i = 1}^{4}\frac{S_{i}}{\Delta t}}}} & {{Eq}.\mspace{14mu}(12)}\end{matrix}$

where Δt is the readout time for a row of the pixel cell.

FIG. 7 shows a schematic diagram of an imaging system 700 in analternate embodiment. The imaging system 700 includes the light source202 and a modulated receiver 702. The light source 202 generates an AMCWsource signal 204 which is reflected from an object 50 within the sceneto generate a reflected signal 206. The reflected signal 206 is receivedat modulated receiver 702. The modulated receiver 702 includes amodulated sensor array in which readout of a pixel can be controlled bychanging a quantum efficiency of the pixel, as shown in FIG. 8.

FIG. 8 shows a pixel 800 of the modulated sensor array, in anembodiment. The pixel includes a PN junction having a depletion layer802 with an N-region 804 at one end of the depletion layer and aP-region 806 at an opposite end of the depletion layer. The quantumefficiency of the pixel 800 is a measure of a responsiveness of thepixel to incoming light and is determined by the width of the depletionlayer 802. The larger the depletion layer 802, the higher the quantumefficiency. The smaller the depletion layer, the lower the quantumefficiency.

The width of the depletion layer 802 is controlled by application of areverse bias voltage 808. As the reverse bias voltage 808 is increased,the depletion layer 802 increases in size, making the pixel 800 moreresponsive to incoming light. As the reverse bias voltage 808 isdecreased, the depletion layer 802 decreases in size, making the pixel800 less responsive to incoming light.

FIG. 9 shows a graph 900 of quantum efficiency for PN-junctions havingdifferent doping concentrations. Reverse bias voltage is shown along theabscissa and quantum efficiency (QE) is shown along the ordinate axis.Each quantum efficiency curve shows an increase as the reverse bias isincreases. The reverse bias voltage can be increased in order toincrease the quantum efficiency of the pixel, as shown, for example, byQE curve 902. At a large enough reverse bias voltage, the QE curve 902rises above a readout threshold 904. The readout period of the pixel cantherefore be controlled by adjusting the reverse bias voltageappropriately.

Returning now to FIG. 7, system controller 214 synchronizes operation ofthe light source 202 and modulated receiver 702. The system controller214 controls the reverse bias voltage at each row of the sensor array inorder to readout rows sequentially, as shown in FIG. 4. The signalprocessor 216 then determines phase delay and amplitude values of thesignal row amplitudes and the image processor 218 determines a depthand/or image of the object, giving a three-dimensional image of theobject.

FIG. 10 shows a block diagram 1000 illustrating operation of thealternate imaging system 700 of FIG. 7. The block diagram 1000 includesthe light source 202, sensor array 210, signal processor 216 and imageprocessor 218 as well as the modulated receiver 702.

The light source 202 generates the source signal 204 having a waveformF_(TX) given by Eq. (13):

F _(TX) =P·[cos(2πf _(s) t−ϕ ₀)+1]  Eq, (13)

where P is the original power of the source signal 204, f_(s) is thefrequency of the source signal, and ϕ₀ is the original phase of thesource signal. A reflected signal 206 created by reflection of thesource signal 204 from the environment or object 50 is incident at thereceiver 208. The reflected signal 206 received at selected pixel (n, m)of the modulated receiver 702 has a waveform F_(RX) ^(n,m) in given byEq. (14):

F _(RX) ^(n,m) =A _(n,m)·[cos(2πf _(s)(t−t _(d) ^(n,m))−ϕ₀)B_(n,m)]  Eq, (14)

where B_(n,m) is an amplitude of an oscillation of the reflected signal206 at the modulated receiver 702 and B_(n,m) is related to an averagepower of the reflected signal 206. The reverse bias voltage 808 of themodulated receiver 702 is modulated to control the quantum efficiency ofthe pixels. The modulation function for the reverse bias voltage 80 andthus for the quantum efficiency is given by Eq. (15):

F _(MIX)=½·[cos(2πf _(MIX) t)+1]  Eq, (15)

where f_(MIX) is the frequency of modulation. The mixing frequencyf_(MIX) is selected to be close to the frequency of the source signalf_(s), both of which are in the Megahertz range. The difference Δfbetween the mixing frequency and the frequency of the source signal isgiven by Eq. (16):

f _(MIX) =f _(s) −Δf  Eq. (16)

and is generally in the range of kilohertz. The modulation of thequantum efficiency of the pixels generates a down-converted signal or asignal having a lower frequency. The main component of thedown-converted mixed signal F_(IN) received at the sensor array 210 isshown in Eq. (17):

$\begin{matrix}{F_{IN}^{n,m} = {\frac{A_{n,m}}{4} \cdot {\cos\left( {{2{\pi\Delta}\; f\; t} - \phi_{d}^{n,m} - \phi_{0}} \right)}}} & {{Eq},\;(17)}\end{matrix}$

Information about the object is held in the various parameters of thewaveform recorded at the sensor array 210. For example, the phase termϕ_(d) ^(n,m) holds information about a depth of the object and intensitymeasurements at the pixel (n, m) helps determined reflectance of theobject.

While the above disclosure has been described with reference toexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from its scope. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the disclosure without departing from the essentialscope thereof. Therefore, it is intended that the present disclosure notbe limited to the particular embodiments disclosed, but will include allembodiments falling within the scope thereof

What is claimed is:
 1. A method of determining a range of an object at avehicle, comprising: transmitting a source signal from the vehicle at asource frequency at the object, wherein the source signal is reflectedfrom the object to create a reflected signal; modulating the reflectedsignal using a mixing frequency to form a down-converted signal;recording the down-converted signal at a sensor array of the vehicle;and determining the range of the object to the vehicle using thedown-converted signal.
 2. The method of claim 1, wherein modulating thereflected signal comprises modulating an intensity of the reflectedsignal at the mixing frequency using a modulator between the object andthe sensor array.
 3. The method of claim 1, wherein modulating thereflected signal comprises modulating a quantum efficiency of the sensorarray at the mixing frequency.
 4. The method of claim 1, furthercomprising recording the down-converted signal at a pixel cellcomprising four rows, each row having four pixels.
 5. The method ofclaim 4, further comprising binning the four pixels of each of the fourrows to obtain four binned pixels, each binned pixel having a row signalamplitude and a time stamp.
 6. The method of claim 5, further comprisingdetermining at least one of: (i) a phase of the reflected signal fromthe row signal amplitudes; and (ii) an amplitude of the reflected signalfrom the row signal amplitudes.
 7. The method of claim 6, furthercomprising determining a time-of-flight of the reflected signal and arange to the object from the time-of-flight.
 8. An imaging system of avehicle, comprising: a light source configured to transmit a sourcesignal at a source frequency at an object, wherein the source signal isreflected from the object to create a reflected signal; a receiverhaving a sensor array, the receiver configured to modulate the reflectedsignal at a mixing frequency to generate a down-converted signal andrecord the down-converted signal at the sensor array; and a processorconfigured to determine a range of the object to the vehicle using thedown-converted signal.
 9. The imaging system of claim 8, furthercomprising a modulator configured to modulate an intensity of thereflected signal at the mixing frequency to generate the down-convertedsignal.
 10. The imaging system of claim 8, wherein a quantum efficiencyof the sensor array is adjusted at the mixing frequency to generate thedown-converted signal.
 11. The imaging system of claim 8, wherein theprocessor is further configured to generate a pixel cell comprising fourrows, each row having four pixels, and record the down-converted signalusing the pixel cell.
 12. The imaging system of claim 11, wherein theprocessor is further configured to bin the four pixels of each of thefour rows to obtain four binned pixels, each binned pixel having a rowsignal amplitude and a time stamp.
 13. The imaging system of claim 12,wherein the processor is further configured to determine at least oneof: (i) a phase of the reflected signal from the row signal amplitudes;and (ii) an amplitude of the reflected signal from the row signalamplitudes.
 14. The imaging system of claim 13, wherein the processor isfurther configured to determine a time-of-flight of the down-convertedsignal and a range to the object from the time-of-flight.
 15. A vehicle,comprising: a light source configured to transmit a source signal at asource frequency at an object, wherein the source signal is reflectedfrom the object to create a reflected signal; a receiver having a sensorarray, the receiver configured to modulate the reflected signal at amixing frequency to generate a down-converted signal and record thedown-converted signal at the sensor array; and a processor configured todetermine a range of the object using the down-converted signal.
 16. Thevehicle of claim 15, further comprising a modulator configured tomodulate an intensity of the reflected signal at the mixing frequency togenerate the down-converted signal.
 17. The vehicle of claim 15, whereina quantum efficiency of the sensor array is adjusted at the mixingfrequency to generate the down-converted signal.
 18. The vehicle ofclaim 15, wherein the processor is further configured to generate apixel cell comprising four rows, each row having four pixels, and recordthe down-converted signal using the pixel cell.
 19. The vehicle of claim18, wherein the processor is further configured to bin the four pixelsof each of the four rows to obtain four binned pixels, each binned pixelhaving a row signal amplitude and a time stamp.
 20. The vehicle of claim19, wherein the processor is further configured to determine at leastone of: (i) a phase of the reflected signal from the row signalamplitudes; and (ii) an amplitude of the reflected signal from the rowsignal amplitudes.